S15
Track-Topic
ESTRO 2026
stability was maintained, however, individual catheters in some patients strongly deviated from their expected positions more than 5 mm and up to 12 mm. Conclusion: Overall, the implant geometry remained largely stable, yet substantial deviations occurred in individual patients and a subset of fractions and catheters. The most affected catheters typically lay close to the chest wall and deviations arose typically at a later stage of the treatment. EMT provides a reliable means to detect such changes and can enable routine screening for relevant anatomical changes as part of enhanced quality assurance in interstitial brachytherapy. Keywords: inter-fractional changes, implant stability, EMT CT-guided HDR brachytherapy for breast cancer liver metastases: Multicenter registry analysis of dosimetric predictors, efficacy, and safety Magdalena Peszy ń ska-Piorun 1 , Przemys ł aw Janiak 1 , Piotr M ęż e ń ski 1 , Lukasz Kuncman 2 , Magdalena Orzechowska 3 , Federico Mastroleo 4 , Sylwia Sroka 5 , Ewa Wojtyna 5 , Konstantinos Ferentinos 6 , Kathrin Hering 7 , Hai Minh Ha 8 , Daniel Medenwald 9 , Matthias P Fabritius 10 , Stefanie Corradini 10 , Max Seidensticker 9 , Jens Ricke 9 , Frederik Fuchs 10 , Paul Rogowski 10 , Jacek Fijuth 2 , Mateusz Bilski 11 1 Radiotherapy Treatment Planning Department, opernicus Memorial Hospital in Lodz Comprehensive Cancer Center and Traumatology, Lodz, Poland. 2 Department of Radiotherapy, Medical University of Lodz, Ł ód ź , Poland. 3 Department of Molecular Carcinogenesis, Medical University of Lodz, Ł ód ź , Poland. 4 Department of Radiation Oncology, Mayo Clinic, Rochester, USA. 5 Department of Medical Physics, Saint Johns Cancer Center, Lublin, Poland. 6 Department of Radiotherapy and Radiation Oncology, German Oncology Center, University Hospital of the European University Cyprus, Limassol, Cyprus. 7 Department of Radiotherapy and Radiation Oncology, University Hospital Leipzig, Leipzig, Germany. Mini-Oral 5226 8 University Clinic for Radiation Therapy, University Hospital Magdeburg A. ö. R, Magdeburg, Germany. 9 Department of Radiology, LMU University Hospital, LMU Munich, Munich, Germany. 10 Department of Radiation Oncology, LMU University Hospital, LMU Munich, Munich, Germany. 11 Department of Radiotherapy, Affidea Nu-med Center of Cancer Diagnostics and Therapy, Zamo ść , Poland Purpose/Objective: To identify dosimetric predictors and assess local efficacy and safety of CT-guided HDR brachytherapy as metastasis-directed therapy for breast cancer liver
Intelligence inMedicine, Elsevier, 2007.
Mini-Oral 5109
Inter-fractional geometric stability of multi- catheter implants during interstitial breast brachytherapy Christopher Dürrbeck 1,2 , Rainer Fietkau 1,2 , Vratislav Strnad 1,2 , Christoph Bert 1,2 1 Department of Radiation Oncology, Universitätsklinikum Erlangen, Erlangen, Germany. 2 Comprehensive Cancer Center, Universitätsklinikum Erlangen, Erlangen, Germany Purpose/Objective: Geometric stability of the catheter implant is crucial for accurate dose delivery in interstitial breast brachytherapy. Despite its clinical importance, the implant stability is not systematically assessed. This longitudinal study quantified inter-fractional geometric changes of multi-catheter implants throughout the treatment course using electromagnetic tracking (EMT). Material/Methods: irradiation or boost treatment after whole breast irradiation were included. Implant geometry was captured using an EMT system providing real-time catheter positions with sub-millimetre accuracy. Measurements were performed in treatment position after CT simulation and after each fraction. To assess inter-fractional changes, EMT-based geometries were rigidly registered to clinical, CT-based reconstructions, which were obtained following institutional and GEC- ESTRO guidelines. Results: Across all patients, catheters and fractions, the median deviation was 2.3 mm (IQR 2.1 mm, 95th percentile 6.9 mm), with individual patients ranging from 0.8 mm to 5.1 mm. Deviations were smallest after the planning CT on the CT couch with a median of 1.6 mm, followed by an increase to 2.2-2.3 mm during the early treatment phase from fraction 1 to 3 (up to 30 hours post-implantation; see Figure 1, top). After 48 hours, i.e., from treatment day 3 onwards, the median deviations stabilise at 2.6-2.9 mm. The interquartile range remained relatively consistent across fractions, whereas the variability between catheters was larger (Figure 1, bottom) since a modest increase was observed for catheters located towards the chest wall. The variability increased as well in this region, 80 breast cancer patients undergoing HDR brachytherapy for accelerated partial breast suggesting greater geometric uncertainty for more peripheral catheters, likely due to local anatomical deformation by the pectoral muscle or the influence of breathing. Overall, it was found that geometric
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